1. Field of the Invention
The present invention relates to elastic-wave filter devices. In particular, the present invention relates to an elastic-wave filter device that includes a band-pass filter and a band-eliminate filter connected to the band-pass filter.
2. Description of the Related Art
Conventionally, elastic-wave filter devices have been used as filter devices mounted on RF circuits for communication equipment, such as mobile phones. An elastic-wave filter device is a filter device that uses elastic waves, such as surface acoustic waves, boundary acoustic waves, and bulk waves. Examples of the elastic-wave filter device include an elastic-wave band-pass filter device capable of passing signals in a certain frequency band, and an elastic-wave band-eliminate filter device capable of rejecting signals in a certain frequency band.
For example, Japanese Unexamined Patent Application Publication No. 2004-129238 discloses an elastic-wave filter device that includes a ladder elastic-wave band-eliminate filter composed of a plurality of parallel-arm elastic wave resonators and at least one series inductor. Japanese Unexamined Patent Application Publication No. 2004-129238 describes a technique in which a stop band is widened by making resonance frequencies of the plurality of parallel-arm elastic wave resonators different from each other in the elastic-wave band-eliminate filter.
Japanese Unexamined Patent Application Publication No. 2004-129238 also describes a technique in which the elastic-wave band-eliminate filter is combined with another elastic wave filter. For example, the elastic-wave band-eliminate filter may be combined with an elastic-wave band-pass filter. FIG. 15 illustrates an example of an elastic-wave filter device that includes an elastic-wave band-eliminate filter and an elastic-wave band-pass filter.
As illustrated in FIG. 15, an elastic-wave filter device 100 includes an elastic-wave band-pass filter 110 and an elastic-wave band-eliminate filter 120. The elastic-wave band-pass filter 110 and the elastic-wave band-eliminate filter 120 are connected in series between an input terminal 101 and an output terminal 102. The elastic-wave filter device 100 has a pass band and a stop band at frequencies higher than the pass band. The pass band is formed mainly by the elastic-wave band-pass filter 110. The stop band is formed mainly by the elastic-wave band-eliminate filter 120. With this configuration, it is possible to increase out-of-band attenuation at frequencies higher than the pass band. As illustrated in FIG. 15, the elastic-wave band-eliminate filter 120 is a ladder filter that includes a first parallel-arm elastic wave resonator P101, a series inductor L100, and a second parallel-arm elastic wave resonator P102.
As described in Japanese Unexamined Patent Application Publication No. 2004-129238, by making a resonance frequency f101 of the first parallel-arm elastic wave resonator P101 different from a resonance frequency f102 of the second parallel-arm elastic wave resonator P102, it is possible to widen the stop band at frequencies higher than the pass band. However, in the elastic-wave filter device 100, it may be difficult to achieve sufficiently large attenuation in the stop band at frequencies higher than the pass band.
As a way to increase attenuation in the stop band, it may be possible to reduce impedances of the first and second parallel-arm elastic wave resonators P101 and P102 included in the elastic-wave band-eliminate filter 120. This is because reducing the impedances of the first and second parallel-arm elastic wave resonators P101 and P102 makes signals flow easily through parallel arms to a ground potential.
However, reducing the impedances of the first and second parallel-arm elastic wave resonators P101 and P102 may increase insertion loss at higher frequencies in the pass band.